Build Darwin libclang_rt libraries against real SDKs.

The minimal fake SDK was very useful in allowing us to build for all
Darwin platforms without needing access to the real SDKs, but it did
not support building any of the sanitizer runtimes. It's important to
fix that. As a consequence, if you don't have the iOS SDKs installed,
we will now skip building the iOS-specific libclang_rt libraries.
rdar://problem/18825276

llvm-svn: 221621